The present invention relates to fuel cell power plants that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to procedures for shutting down an operating fuel cell system.
Fuel cell power plants are well-known and are commonly used to produce electrical energy from hydrogen containing reducing fluid and process oxidant reactant streams to power electrical apparatus such as power plants and transportation vehicles. In fuel cell power plants of the prior art, it is well known that, when an electrical circuit connected to the fuel cells is disconnected or opened and there is no longer a load across the cell, such as upon and during shut down of the cell, the presence of air on a cathode electrode along with hydrogen fuel remaining on an anode electrode, often cause unacceptable anode and cathode potentials, resulting in oxidation and corrosion of catalyst and catalyst support materials and attendant cell performance degradation.
Efforts have been proposed to return the cathode electrode to a passive, non-oxidative state upon shut down of the fuel cell. For example, it was thought that inert gas needed to be used to purge both the anode flow field and the cathode flow field immediately upon cell shut down to passivate the anode and cathode electrodes so as to minimize or prevent such cell performance degradation. Further, the use of an inert gas purge avoided, on startup, the possibility of the presence of a flammable mixture of hydrogen and air, which is a safety issue. While the use of 100% inert gas as the purge gas is most common in the prior art, commonly owned U.S. Pat. Nos. 5,013,617 and 5,045,414 describe using 100% nitrogen as the anode side purge gas, and a cathode side purging mixture comprising a very small percentage of oxygen (e.g. less than 1%) with a balance of nitrogen. Both of these patents also discuss the option of connecting a dummy electrical load across the cell during the start of a purging process to lower the cathode potential rapidly to between the acceptable limits of 0.3-0.7 volt.
A solution has been proposed that avoids the costs associated with storing and delivering a separate supply of inert gas to fuel cells. The costs and complexity of such stored inert gases are undesirable especially in automotive applications where compactness and low cost are critical, and where the system must be shut down and started up frequently. That solution includes shutting down a fuel cell power plant by disconnecting the primary electricity using device (hereinafter, xe2x80x9cprimary loadxe2x80x9d), shutting off the air or process oxidant flow, and controlling the fuel flow into the system and the gas flow out of the system in a manner that results in the fuel cell gases coming to equilibrium across the cells, with the fuel flow shut off, at a gas composition (on a dry basis, e.g. excluding water vapor) of at least 0.0001% hydrogen, balance fuel cell inert gas, and maintaining a gas composition of at least 0.0001% hydrogen (by volume), balance fuel cell inert gas, during shut down. Preferably, any nitrogen within the equilibrium gas composition is from air either introduced into the system directly or mixed with the fuel. This method of fuel cell shut down also includes, after disconnecting the primary load and shutting off the air supply to the cathode flow field, continuing to supply fresh fuel to the anode flow field until the remaining oxidant is completely consumed. This oxidant consumption is preferably aided by having a small auxiliary load applied across the cell, which also quickly drives down the electrode potentials. Once all the oxidant is consumed the fuel feed is stopped, a fuel exit valve is shut, and air is introduced into the anode flow field (if needed) until the hydrogen concentration in the anode flow field is reduced to a selected intermediate concentration level, above the desired final concentration level. Air flow into the anode flow field is then halted, and the fuel cell gases are allowed to come to equilibrium, which will occur through diffusion of gases across the electrolyte and chemical and electrochemical reaction between the hydrogen and the added oxygen.
An intermediate hydrogen concentration level is selected based upon the relative volumes of the anode and cathode flow fields, such that the resulting hydrogen concentration at equilibrium (i.e. after all the oxygen has been consumed and the hydrogen and fuel cell inert gases are fully dispersed throughout the cell) will be within a desired range. Thereafter, during continued shut-down, a hydrogen concentration is monitored; and hydrogen is added, as and if necessary, to maintain the desired hydrogen concentration level. That shut down method urges that a desired range of hydrogen concentration is between 0.0001% and 4%, with the balance being fuel cell inert gases. The latter step of adding hydrogen is likely to be required due to leakage or diffusion of air into the fuel cell and/or leakage or diffusion of hydrogen out of the fuel cell, such as through seals. As air leaks into the system, hydrogen reacts with the oxygen in the air and is consumed. The hydrogen needs to be replaced, from time to time, to maintain the hydrogen concentration within the desired range.
Known improvements to the problem of oxidation and corrosion of electrode catalysts and catalyst support materials have reduced the deleterious consequences of the presence of oxygen on the cathode electrode and a non-equilibrium of reactant fluids between the anode and cathode electrodes that result in unacceptable anode and cathode electrode potentials upon and during shut down of a fuel cell. However, during the time it takes for an adequate amount of hydrogen to diffuse through the electrolyte from the anode flow field to the cathode flow field to achieve a hydrogen concentration equilibrium in both flow fields, an unacceptable potential exists at the cathode electrode leading to unwanted oxidative deterioration of the cathode catalyst and catalyst support materials.
Consequently, there is a need for a fuel cell power plant that does not cause significant performance degradation of the plant, and that minimizes oxidation and corrosion within plant fuel cells at shut down of the plant, during shut-down, or upon restarting the fuel cell power plant.
The invention is a system and method for shutting down a fuel cell power plant. The system for shutting down the fuel cell power plant includes at least one fuel cell for generating electrical current from hydrogen containing reducing fluid and process oxidant reactant streams. The fuel cell includes an anode electrode and a cathode electrode on opposed sides of an electrolyte; an anode flow field adjacent the anode electrode for directing the reducing fluid stream to flow adjacent the anode electrode; and a cathode flow field adjacent the cathode electrode for directing the process oxidant stream to flow adjacent the cathode electrode. A cathode inlet valve and a cathode outlet valve are secured to cathode inlet and exhaust lines in fluid communication with the cathode flow field for permitting and terminating flow of the process oxidant stream through the cathode flow field. An external circuit is connected to the anode and cathode electrodes for conducting the electrical current generated by the fuel cell, and a primary load is connected through a primary load switch to the external circuit. An auxiliary load is connected through an auxiliary load switch to the external circuit, and a power supply is connected through a power supply switch to the external circuit.
The fuel cell power plant may be controlled so that whenever the primary load switch disconnects the primary load from receiving the electrical current and the cathode inlet and outlet valves terminate flow of the process oxidant through the cathode flow field, the auxiliary load switch connects the auxiliary load to receive any electrical current from the fuel cell to consume oxygen remaining within the cathode flow field, and the power supply switch connects the power supply to the external circuit to increase a concentration of hydrogen within the cathode flow field. By applying the electrical power supply to the anode and cathode electrodes, the electrodes and electrolyte are effectively turned into a hydrogen pump, wherein the hydrogen fuel dissociates at the anode electrode into electrons and hydrogen ions, the hydrogen ions pass through the electrolyte to the cathode electrode as in normal fuel cell operation, and the electrons flow through the power supply to the cathode electrode to evolve hydrogen at the cathode electrode in the absence of oxygen. Therefore, application of the power supply across the cell significantly decreases an amount of time necessary to achieve equilibrium of hydrogen concentrations within the anode and cathode flow fields.
In order to minimize a risk of the hydrogen concentration within the cathode fuel cell becoming a flammable concentration as air diffuses into the cathode flow field during shut down of the plant through leaks, or upon start up of the plant as air is blown through the cathode flow field, a ventilation enclosure and ventilation fan may be included as part of the system.
The system may be utilized as a method for shutting down a fuel cell power plant through the steps of: disconnecting the primary load switch so that the primary load ceases receiving the electrical current from the fuel cell; terminating flow of the process oxidant through the cathode flow field; connecting the auxiliary load switch so that the auxiliary load receives any electrical current generated by the fuel cell to consume oxygen remaining within the cathode flow field; disconnecting the auxiliary load switch whenever oxygen remaining within the fuel cell has been consumed; connecting the power supply switch so that electrical power from the power supply flows to the anode and cathode electrodes to increase a concentration of hydrogen within the cathode flow field; and, then, decreasing or eliminating flow of the hydrogen containing reducing fluid into the anode flow field after an equilibrium gas concentration of at least 0.0001% hydrogen, balance fuel cell inert gases is achieved in both the anode and cathode flow fields while the fuel cell power plant is shut down.
In a further embodiment, the concentration of hydrogen within the anode and cathode fuel cells may be monitored, and fed into the flow fields as necessary to remain within a concentration range of between 0.0001% and 4.0% during shut down of the plant as hydrogen is consumed by any oxygen leaking into the flow fields. Additionally, the method may also include admitting air to pass into the anode flow field to avoid creation of a partial vacuum.
Accordingly, it is a general purpose of the present invention to provide a system and method of shutting down a fuel cell power plant that overcomes deficiencies of the prior art.
It is a more specific object to provide a system and method of shutting down a fuel cell power plant that hastens production of an equilibrium hydrogen concentration between anode and cathode flow fields to thereby passivate the cathode catalyst and catalyst support material.
It is yet another object to provide a system and method of shutting down a fuel cell power plant that minimizes oxidative deterioration of anode and cathode catalysts and catalyst support materials making up the anode and cathode electrodes.
It is another object to provide a system and method of shutting down a fuel cell power plant that minimizes oxidative deterioration without usage of stored inert gases to purge anode and cathode flow fields.
These and other objects and advantages of the present system and method of shutting down a fuel cell power plant will become more readily apparent when the following description is read in conjunction with the accompanying drawing.